Inked mice hint at how tattoos persist in people

Immune cells pass pigment from one generation to the next

TAIL TATS When researchers tattooed mouse tails, the only cells found with ink inside were macrophages. The tattoo appeared the same before (left) and after (right) ink-holding macrophages were killed, because it was recaptured by new macrophages.

Tattoos may have staying power because of a hand off between generations of immune cells known as macrophages, say a group of French researchers.

If true, this would overturn notions that tattoo ink persists in connective tissue or in long-lasting macrophages.

Immunologist Sandrine Henri of the Immunology Center of Marseille-Luminy, in France, and colleagues tattooed mice tails with green ink to see how waste-disposing macrophages in the skin would respond.

“Macrophages will scavenge everything. That’s their job,” Henri says. “If they could do their job properly, tattoo ink would be removed rapidly.”

In the experiment, described March 6 in the Journal of Experimental Medicine, macrophages gobbled up the ink as expected, but did not digest and remove it. Instead, the cells held onto the ink until the researchers killed the cells. About 90 days later, new macrophages moved in and reabsorbed the ink. This capture-release-recapture cycle was key to preserving the tattoos, the researchers say.

But a mouse study doesn’t settle the science of tattoos in humans, says Desmond Tobin, a dermatology expert at the University of Bradford in the United Kingdom, who was not involved in the study. Macrophages may live longer in humans than in mice, and the persistence of those cells might be responsible for preserving tattoos in human skin, he says.

The findings may still help improve tattoo removal, the researchers say. Combining laser therapy with a treatment to get rid of skin macrophages could oust the ink.

Dye another day

On the left, macrophages contain green tattoo ink. The pigment is released when the cells are killed (middle). Within 90 days, new macrophages have recaptured the pigment.